Current work in the field of membrane structure has shown that some constituent protein molecules traverse the entire thickness of the membrane. This raises the possibility of studying the enzymatic function of such molecules with the use of exogenous substrate. In this connection, it is noteworthy that the isolated, intact gastric mucosa catalyzes certain reactions. Thus we have shown that p-nitrophenyl phosphate and ATP are hydrolyzed when these substrates are added to the physiological salt solutions bathing the intact mucosa, mounted between chambers. As substrate disappears, appropriate amounts of product appear in these solutions and thus are readily available for measurement at timed intervals. A particularly vigorous ATPase can be demonstrated by including ATP in the nutrient (serosal) bathing solution. The proposed research is directed towards the detailed exploration of the surface ATPase and its relationship to the mechanisms determining electrical and secretory activity of the mucosa. In this manner the surface ATPase may act as a probe to reflect the handling of endogenous ATP by membrane-bound ATPases involved in active transport by the oxyntic cell of the stomach.